1. Field of the Invention
The present invention relates to a testing method for an optical fiber, which detects problems in the optical fiber based on the light pulse reflected by the optical fiber line.
This application is based on patent application No. Hei 09-081426 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
OTDR (Optical Time Domain Reflectometry) is a method which measures the loss of connection or detects a trouble point in an optical fiber to be tested, by sending the light pulse to the optical fiber and detecting the returning light.
FIG. 7 is a block diagram showing an apparatus using a conventional OTDR construction. The apparatus contains a timing generator 51 which determines the timing of generation of a light pulse, driver circuit 52, light source 53 which outputs the light pulse synchronizing with the signal generated by the timing generator 51, optical directional coupler 54 at which incident light from A goes out of B to the optical fiber 59 to be tested and incident light from the optical fiber 59 through B goes out of C. The apparatus also contains a light receiver 55 which converts the outgoing light from the optical directional coupler 54 to the electric signal and amplifier 56 which amplifies the electric signal output by the light receiver 55. The apparatus also contains a digital processor 57 which digitizes and processes the timing signal output by the timing generator 51, the electric signal is amplified by the amplifier 56, and the result is displayed on the indicator 58.
In the example shown in FIG. 7, the driver circuit 52 generates the pulse current based on the timing signals from the timing generator 51, causing the light source 53 to emit light. The light emitted by the light source 53 goes through the optical directional coupler 54 and enters the optical fiber 59 to be tested.
Backscattered light or light returning by reflection is transmitted to the light receiver 55 via the optical directional coupler 54. The lights are converted into electrical signals and are amplified by the amplifier 56. The backscattered light returning from the optical fiber 59 to be tested is caused by Rayleigh scattering in the optical fiber 59.
The electrical signal amplified by the amplifier 56 is converted into a digital signal by the digital processor 57, and processes noise reduction such as equalization. Afterward, the above result is converted logarithmically and is displayed on the indicator 58.
FIG. 8 shows a resulting waveform of measurement of optical fiber 59 to be tested which is serially connected to optical fiber 59A with optical fiber 59B using OTDR with a construction shown in FIG. 7.
In FIG. 8, the x-axis represents a distance (equal to the time after the light emitted by the light source 53 reaches the optical fiber 59 to be tested), and the y-axis represents the intensity of the received light.
As shown in FIG. 8, the line goes down linearly as the distance on the x-axis increases because the signal is logarithmically converted at the digital processor 57. Also in FIG. 8, the loss at the connection appears as a non-linear step on the curve, and the reflection by the connectors or terminations is observed as a large discontinuous upward waveform.
There is a problem in the conventional method in that a loss at a connection by fusion or a loss caused by a bend at a certain point in the optical fiber to be tested are observed as a non-linear step on the graph. Detecting a loss depends on finding the non-linear steps mentioned above; however, this is not easy since there are numerous waveforms from the optical fiber.
FIG. 9 is a sample loss distribution of a optical fiber observed with an OTDR method. As shown in FIG. 9, the curve of the loss distribution does not generally show a constant smoothness because of non-uniformity in the longitudinal direction of the optical fiber. For example, it shows a characteristics curve enclosed by the dashed line.
In the ambiguous case such as is shown in FIG. 9, the existence of a non-linear step on the curve is found by a skilled observer.
However, the objectives of the methods mentioned above are to detect the connection point even if noise is superimposed on the curve of the signal, and are not for the purpose of detecting the connection point in the curve of loss distribution, which is observed as a non-uniform linear attenuation curve, in an optical fiber to be tested.